Data communications between personal computers, computer terminals and peripheral equipment is desired for many business computing applications. A common application for data communications is a local area network. In a typical local area network, data communication is carried out over a hard wired media, usually using coaxial or twisted pair cable. Staffing facility or assignment changes require re-routing these data communication cables which is costly in both manpower and downtime for rewiring. Similar requirements apply to applications which link computers with computer terminals.
Infrared optical energy has been used for data communications. Earlier uses of optical energy for communication were restricted either to point-to-point or multipoint communication links or spatially diffuse transmission of infrared power to wide field of view receptors employing either direct or diffusely reflected optical paths. Point-to-point or multipoint communication lengths require either fiber optic cable or direct free-space aimed transmission. Fiber optic cable communications networks require consultants with specialized cabling skills and present the same system re-routing difficulties as the hard wired communications systems described above. Direct free-space aimed transmissions require a straight line path between a transmitter and receiver which is transparent to the communications media. An example of such a direct free space aimed transmission is remote control of a home television receiver. The remote control unit must be aimed at the receiver of the television in order to activate any kind of control.
Spatially diffuse optical systems using infrared radiation have been disclosed by Gfeller and Bapst of IBM in their "Wireless In-House Data Communication Via Diffused Infrared Radiation", proceedings of the IEEE, Vol. 67, No. 11, November 1979. This system requires that the communications area be flooded with radiation containing communicated information which requires large amounts of power to achieve reliable communications at high speeds. Furthermore, this system does not permit simultaneous co-existence of more than one independently operating communications network in the same frequency range of radiation.
FIG. 1 shows a schematic representation of the system suggested by Gfeller and Bapst. The communications area 100 contains four computer terminals 102, 104, 106 and 108 and a satellite transponder 110. Each of the terminals 102 through 108 is equipped with an infrared transceiver having a transmitter and a receiver. The satellite transponder 110 is also equipped with an infrared transmitter and receiver. The satellite transponder 110 is hard wire coupled to a controller 112. The controller is hard wire coupled to a host computer 114. As depicted in FIG. 1, the satellite transponder floods the communications area 100 with infrared radiation 116. The terminal 102 is configured in the receive mode. Any infrared radiation 116 impinging into the wide field of view represented by cone 118 is sensed by the terminal 102. Terminal 106 is shown in transmit mode. The infrared radiation 120 is transmitted by terminal 106 into the room 100. The satellite transponder 110 senses the infrared radiation 120 transmitted by terminal 106. For terminal 106 to communicate with another terminal 102 in the room, the satellite transponder must receive the communication from terminal 106 through infrared radiation 120 as shown and flood the room with infrared radiation 116 in order for terminal 102 to receive the infrared radiation within its field of view 118.
The satellite transponder as described above must be hard wired into a controller 112. The controller 112 is controlled by a host computer 114. For multiple communications networks to coexist within a single communications area 100, the terminal transceivers and their associated satellite transponder for each separate network must operate at a different frequency requiring specialized hardware for each network.
Other wireless communications systems employ radio frequency signals conducted by power lines or by free space transmission. Power line conductive signals are prone to unpredictable range and high error rates due to noise caused by other equipment attached to the power line. Free space radio based systems are subject to wide variations in signal strength due to multi-path reflections. Radio based systems are subject to interference. Information transmitted on a radio based system can be detected from a source external to the communications area. Further, radio based services are subject to government regulation.
Wireless data communication is desirable to avoid the expense and loss of productivity associated with wiring and rewiring of communication paths. A wireless data communications link is needed which overcomes the problems of flooding the communications area with radiation, wiring of transponders and radio signal communications.